It is really a great question, especially if we also look at the differences between an astronomical season and a meteorological season.
Each season has both an astronomical start and a meteorological start. It sounds complicated, but trust me, it really isn’t. The astronomical start date is based on the position of the Sun in relation to the Earth, while the meteorological start date is based on the 12-month calendar and the annual temperature cycle.
Many folks think that some parts of the year are hotter because we're nearer to the Sun, but the real reason is that the Earth is wonky. “Wonky,” is a highly technical and scientific term, so I’ll explain. Our planet is slightly tilted, and our four seasons are caused by three dominant variables-- the movement of the Earth around the sun, the tilt of the Earth, and how high the sun will get in the sky.
The earth moves two ways. It spins and it moves around the sun. The spinning of the earth is called rotation and takes about 24 hours. At the same time, the earth is moving in its orbit around the sun. This orbit is called a revolution. It takes a little over 365 days.
But neither the earth’s rotation nor its distance orbit causes our seasons. The tilt of the earth is the main reason for our seasons. This tilt will change how high the sun will get in the sky and that causes our seasons. When the sun is higher in the sky, it will be summer. The higher the sun gets, the more it can warm the earth. The time of year when we have winter is when the sun does not get nearly as high in the sky. This is because the North pole always points the same way as the Earth revolves around the Sun. When the Earth's axis points towards the Sun, it is summer for that hemisphere. When the Earth's axis points away, it is winter.
The 2020 dates:
- Spring – Astronomical Spring began on March 19th at 11:50pm EDT. Meteorological Spring started on March 1st.
- Summer - Astronomical Summer begins on June 20th at 5:44pm EDT. Meteorological Summer starts on June 1st.
- Fall - Astronomical Fall begins on September 22nd at 9:31am EDT. Meteorological Fall starts on September 1st.
- Winter - Astronomical Winter begins on Monday, December 21st at 5:02am EDT. Meteorological Winter starts on December 1st.
What exactly is a “season?” Astrologists and meteorologists define seasons differently.
The astronomical start of a season is based on the position of the Earth in relation to the Sun. A solstice is when the Sun reaches the most southerly or northerly point in the sky, an equinox is when the Sun passes over Earth’s equator. Because of leap years, the dates of the equinoxes and solstices can shift by a day or two over time, causing the start dates of the seasons to shift, too.
In contrast, the meteorological start of a season is based on the annual temperature cycle and the 12-month calendar. According to this definition, each season begins on the first of a particular month and lasts for three months. Climate scientists and meteorologists created this definition to make it easier to keep records of the weather, since the start of each meteorological season doesn’t change from year to year.
Temperate regions of Earth experience four seasons because of shifting sunlight, which is determined by how the Earth orbits the Sun and the tilt of our planet’s axis and not how close or far from the sun the planet is.
“Are seasons all the same length?” is usually the next question.
While it can sometimes feel like winter is dragging on forever (for gardeners at least) it is actually the shortest season of the year. In the Northern Hemisphere, that is, as everything is opposite in the Southern Hemisphere – our summer is their winter and our spring is their fall.
Thanks to the elliptical shape of Earth’s orbit around the Sun, the Earth doesn’t stay the same distance from the Sun year-round. In January, we reach the point in our orbit nearest to the Sun (called perihelion), and in July, we reach the farthest point (aphelion).
When the Earth is closer to the Sun, the star’s gravitational pull is slightly stronger, causing our planet to travel just a bit faster in its orbit. For those of us in the Northern Hemisphere, this results in a shorter fall and winter, since we are moving faster through space during that time of the year. Conversely, when Earth is farthest from the Sun, it travels more slowly, resulting in a longer spring and summer. In other words, it takes Earth less time to go from the autumnal equinox to the vernal equinox than it does to go from the vernal equinox to the autumnal equinox. Due to all of this, the seasons range in length from about 89 days to about 94 days.
To sum things up, over the course of a year, the Earth goes on a journey around the Sun. The reason we have seasons is because, during its journey around the Sun, the Earth is tilted. The Earth’s tilt affects the amount of daylight each hemisphere gets, which in turn makes the temperature hotter or colder.
For example, here in the northern hemisphere – that’s north of the equator, like in Europe, USA, or India – winter takes place in December, January and February. That’s when the northern hemisphere is tilted away from the Sun, and the days are shorter. For anywhere south of the equator, such as Australia or Latin America, it’s summer during these months. That’s because the southern hemisphere is tilted toward the Sun, and the days are longer.
Every season has a middle point. In summer and winter, these midpoints are called solstices. The summer solstice is the longest day, and shortest night, of the year. The winter solstice is the shortest day of the year, and the longest night.
In spring and autumn, the midpoints are called the equinoxes. At the spring and autumn equinoxes, day and night are just about the same length.
One fun fact in all this. Despite the tilt of the earth, or rather because of its tilt, one star is consistently above the North Pole. It’s called the North Star (Polaris), and while all the other stars and sky objects move about, the North Star stays firmly in the same place in the sky, hence its usefulness in navigation. The North Star is located between the constellations of Cassiopeia and the Big Dipper.
During this time of quarantine, perhaps a useful hobby to pick up is astronomy and the learning of how the constellations and other sky objects move about the sky during the year. You may find that sharing the hobby of Astronomy with your children may be particularly rewarding.
Be safe, be strong, remember to practice social distancing and self-isolation because separately we can all beat this virus together!